The present invention relates to a data transmission method and a data transmission system capable of encoding data with a width of n bits into data with a width of m bits (2n>m>n) and transmitting the resultant data with the width of m bits via m signal lines at a high transmission rate.
It is known to use a differential signal to transmit data without generating radiation noise and without encountering a significant problem associated with common-mode noise.
For example, National Publication of Translated Version No. 9-507978 discloses a high-speed transport system including a differential transmitter of the non-coding type for transmitting a differential signal, a differential receiver for receiving a differential signal, a balanced channel for transmitting the differential signal from the differential transmitter to the differential receiver, and an equalizer for reducing the timing jitter of the differential signal transmitted via the balanced channel.
Referring to a diagram shown in FIG. 7, a conventional parallel data transmission method using a differential signal is described.
In FIG. 7, a data transmission system 30 transmits data with a width of, for example, four bits from a transmitter 32 to a receiver 34 via an eight signal lines (four pairs of signal lines). The transmitter 32 includes four differential drivers 36, each of which transmits one bit of data with the width of four bits, in the form of a differential signal expressed as a potential difference between two signal lines. The receiver 34 includes four differential receiver units 38, each of which receives the differential signal transmitted from a corresponding differential driver, and acquires data from the differential signal expressed as the potential difference between a pair of signal lines.
Transmission of data using differential signals in the above-described manner allows a reduction in radiation noise and a reduction in an influence of common-mode noise.
However, in the conventional differential transmission method, one bit is transmitted using two signal lines, and thus a large number of signal lines are needed to transmit parallel data. This problem is serious in particular when the parallel data has a large bit width.
The number of driver circuits that drive respective pairs of signal lines increases with increasing width of a bus. In transmission of data via a bus with a large width, a great variation can occur in the total value of currents flowing through respective signal lines at transition of data, even if data is transmitted using the differential transmission method. When data is transmitted at a high frequency via a bus, cross coupling between signal lines, fluctuations in the power supply voltage or the ground voltage, electromagnetic radiation due to spike noise, etc. can occur at high-to-low transitions or at low-to-high transitions of data transmitted via the signal line pairs. This problem can occur regardless of the type or the location of the bus, that is, the problem can occur for any bus such as a bus extending on an IC (Integrated Circuit) chip, a bus extending between a chip and another chip on a printed circuit board, or a bus extending between a transmitting device and a receiving device.